Abstract

Abstract. In recent years, intense haze events in megacities such as Beijing have received significant attention. Although secondary organic aerosol (SOA) has been identified as a major contributor to such events, knowledge of its sources and formation mechanisms remains uncertain. We investigate this question through the first field deployment of the extractive electrospray ionisation time-of-flight mass spectrometer (EESI-TOF) in Beijing, together with an Aerodyne long-time-of-flight aerosol mass spectrometer (L-TOF AMS). Measurements were performed during autumn and winter 2017, capturing the transition from non-heating to heating seasons. Source apportionment resolved four factors related to primary organic aerosols (traffic, cooking, biomass burning, and coal combustion), as well as four related to SOA. Of the SOA factors, two were related to solid fuel combustion (SFC), one to SOA generated from aqueous chemistry, and one to mixed/indeterminate sources. The SFC factors were identified from spectral signatures corresponding to aromatic oxidation products, while the aqueous SOA factor was characterised by signatures of small organic acids and diacids and unusually low CO+/CO2+ fragment ratios measured by the AMS. Solid fuel combustion was the dominant source of SOA during the heating season. However, a comparably intense haze event was also observed in the non-heating season and was dominated by the aqueous SOA factor. During this event, aqueous chemistry was promoted by the combination of high relative humidity and air masses passing over high-NOx regions to the south and east of Beijing, leading to high particulate nitrate. The resulting high liquid water content was highly correlated with the concentration of the aqueous SOA factor. These results highlight the strong compositional variability between different haze events, indicating the need to consider multiple formation pathways and precursor sources to describe SOA during intense haze events in Beijing.

Highlights

  • Atmospheric aerosols negatively affect human health (Liu et al, 2017a; Krapf et al, 2017; Beelen et al, 2014; Laden et al, 2006; Pope et al, 2002), visibility (Chow et al, 2002), and urban air quality (Fenger, 1999; Mayer, 1999) on local and regional scales

  • Tong et al.: Quantification of SOA sources in winter in Beijing from aerosol mass spectrometer (AMS) and EESI-TOF measurement is directly emitted from sources such as fossil fuel combustion, industrial emissions, biomass burning, and cooking emissions, or secondary organic aerosol (SOA), which is produced by atmospheric oxidation of volatile organic compounds (VOCs), yielding lower volatility products that can subsequently partition to the particle phase

  • Organic aerosol (OA) sources were investigated in Beijing during an intensive field deployment of AMS and EESI-TOF instruments from late September to mid-December 2017, covering the transition from the non-heating to heating seasons

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Summary

Introduction

Atmospheric aerosols ( known as particulate matter, PM) negatively affect human health (Liu et al, 2017a; Krapf et al, 2017; Beelen et al, 2014; Laden et al, 2006; Pope et al, 2002), visibility (Chow et al, 2002), and urban air quality (Fenger, 1999; Mayer, 1999) on local and regional scales. To reduce ionisation-induced fragmentation, several semi-continuous measurement techniques have been developed, e.g. the Thermal Desorption Aerosol GC/MS-FID (TAG) by Williams et al (2006) and the Filter Inlet for Gases and AEROsols chemical ionisation time-of-flight mass spectrometer (FIGAERO-CIMS) by Lopez-Hilfiker et al (2014) These instruments have lower thermal decomposition and better chemical resolution, like offline filter sampling, they are subject to reaction/vaporisation processes on the collection substrate and decreased time resolution. The extractive electrospray ionisation time-offlight mass spectrometer (EESI-TOF) utilises a soft ionisation technique with minimal thermal energy transfer to the analyte molecules This yields online, near-molecular-level measurements (i.e. molecular formulae) of organic aerosol composition with high time resolution (seconds) without thermal decomposition or ionisation-induced fragmentation (Lopez-Hilfiker et al, 2019). An integrated source apportionment analysis of AMS and EESI-TOF data is performed to characterise the sources and physicochemical processes governing SOA composition

Measurement campaign
Instrumentation
Source apportionment technique
Bootstrap analysis
Campaign overview
AMS source apportionment
Investigation of factor composition by EESI-TOF
POA factors
MO-OOASFC
LO-OOASFC
MO-OOAaq
LO-OOAns
Atmospheric implications
Conclusions
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